A simple converging lens can image a beam of light rays onto a photodiode array. Beams which are incident on the converging lens at a large angle in relation to the optical axis of the converging lens impinge on the photodiode array in the image plane at a greater distance from the optical axis. The greater the distance from the optical axis in the image plane, the greater the imaging aberration (field curvature) becomes. The beam is now no longer focused at one point in the image plane but instead imaged as an elliptical spot.
Hence, as a rule, it is not possible in the case of relatively large photodiode arrays to obtain focused imaging even on photodiodes that are far away from the optical axis. This leads to a deterioration of the resolution in the edge regions since a certain angular segment is then imaged on two photodiodes.
In general, imaging aberrations can be compensated by complicated optics, for example, multi-lens-element objective lenses.